This invention is related generally to a method of making a semiconductor device and specifically to photolithographic methods for forming submicron features including an added process step to harden photoresist material to prevent pattern collapse.
The semiconductor industry has progressively reduced the size of components and connectors on integrated circuits in the pursuit of increased computational power and device speed. State of the art semiconductor devices are approaching the limit of feature sizes that may be formed using conventional photolithography fabrication methods. One of the limits being approached involves the minimum dimension of photoresist structures that can be used during fabrication.
Photolithography employs photoresist to create a patterned structure that protects the underlying surface from subsequent fabrication steps, such as chemical etching. There are two types of photoresists in common use, positive resists and negative resists. Positive resists are sensitized when exposed to ultraviolet light so that exposed areas will dissolve in a developer solution leaving behind unexposed areas. Negative resists are hardened by exposure to ultraviolet light so exposed areas are inhibited from being dissolved by the developer solution while unexposed areas are dissolved.
Using the example of a positive resist process, a conventional photolithography method for producing lines is illustrated in FIGS. 1A and 1B. Supported by a substrate 1 is provided a material layer that forms a surface 2 in which it is desired to form a first and second line. A positive photoresist layer 3 is formed over the surface 2. A first region 5 and a second region 7 in the photoresist layer 3 are simultaneously exposed to electromagnetic radiation 8, such as ultraviolet or actinic light, through openings 11 and 13 in a mask or reticle 9, as illustrated in FIG. 1A. The mask 9 comprises a pattern of lines of opaque material 10, which prevent transmission of light 8, and transparent openings 11, 13. The terms mask and reticle are used interchangeably in the semiconductor arts, with the term reticle often referring to a mask used in step and repeat exposure systems. The photoresist layer 3 is then developed wherein the exposed regions 5 and 7 are removed (when employing a negative resist, the unexposed areas are removed), while the unexposed region 6 remains, as illustrated in FIG. 1B. A gas or liquid etching medium is then permitted to reach the underlying surface 2 through the openings 15, 17 in the photoresist layer 3 to etch lines 16, 18 in surface 2, which are separated by an space distance 19, as illustrated in FIG. 1C.
In the developing step, the exposed areas of a positive photoresist are removed by a developer solution to leave the desired pattern image on the surface. At the end of the developing step, the surface must be rinsed to stop the developing reaction and remove the developer solution from the surface. Typical positive resist developer solutions are alkaline solutions diluted with water, which require only a water rinse. Negative resist developer solutions are organic solvents, which require rinsing with other organic solvents (e.g. n-butlyl acetate). After rinsing, the substrate is dried in preparation for further processing.
Developed, rinsed and dried photoresist layers are sometimes then treated with ultraviolet radiation to reduce the tendency of the photoresist to flow during subsequent process steps where the photoresist will experience high temperatures, which may including bake cycles, plasma etching, ion implantation and ion milling. This treatment is typically accomplished by irradiating the dried photoresist layer with deep UV (e.g.  less than 320 nm) while heating the layer to a high temperature (e.g. 120-190xc2x0 C.) for approximately a minute. Alternatively, developed, rinsed and dried photoresist layers are sometimes then treated by irradiating the surface with electron beams with energies of 1-100KeV.
As the width and spacing of circuit structures is reduced, the width of the photoresist structures used to create them must be reduced. A practical limit being approached in semiconductor feature sizes results from the photoresist structures becoming so narrow with respect to the photoresist layer thickness that they lack the structural rigidity to withstand the forces induced by surface tension of liquid between them when the surface is dried. As feature sizes are reduced, the spacings between opaque regions 30 on the mask 24 are reduced, which results in exposed regions 25, 27, that are illuminated by light 28, and unexposed regions 26 both having narrow widths, as shown in FIG. 2A. When developed, the photoresist features 26 are thin to provide a narrow interline distance 34, and are closely spaced to make the photoresist openings 35, 37 narrow, as illustrated in FIGS. 2A and 2B. As illustrated in FIG. 2C, as the photoresist pattern layer dries, a meniscus 31, 32, 33 of developer or rinse solution forms in the narrow spaces 35, 37 between adjacent photoresist structures 26, 38, 39, which pulls the structures together due to surface tension. Thin structures of relatively weak photoresist material can collapse under such meniscus forces, as illustrated by photoresist structures 38 and 39, which renders the pattern on the surface unusable. Furthermore, thin photoresist structures may collapse under capillary forces of the liquid within spaces during spin developing or spin rinsing, which involves rapidly revolving a wafer while depositing a solution on the wafer near the axis of revolution so the solution is distributed over the wafer by centripetal force. Thus, the prior art methods of photolithography cannot form structures below a critical line dimension and inter-line spacing which are limited by the mechanical strength of the photoresist.
According to one aspect of the present invention, there is provided a method of forming a photoresist layer, comprising the steps of providing a surface, depositing a photoresist layer on the surface, the photoresist layer having material properties, exposing the photoresist layer through a mask to create an exposed area of photoresist and an unexposed area of photoresist, developing the photoresist layer; and applying a stabilizing treatment to the photoresist layer while the photoresist layer is immersed in a liquid to change the material properties.
According to another aspect of the present invention, there is provided a method of making a semiconductor device, comprising the steps of forming at least one semiconductor device on a substrate, forming an insulating layer over the semiconductor device, forming a photoresist layer over the insulating layer, the photoresist layer having material properties, exposing the photoresist layer through a mask to create an area of exposed photoresist and an area of unexposed photoresist, developing the photoresist layer to form an opening in the photoresist layer, applying a stabilizing treatment to the photoresist layer while the photoresist material is immersed in a liquid to change the material properties, forming a trench in the insulating layer; and forming a conductive line in the trench. Alternatively, another aspect of the present invention comprises forming a conductive layer of conductive material on a substrate, forming a photoresist layer over the conductive layer, the photoresist layer having material properties, exposing the photoresist layer through a mask to form a line of exposed photoresist and an area of unexposed photoresist, developing the photoresist layer to remove the area of unexposed photoresist, applying a stabilizing treatment to the photoresist layer while the photoresist material is immersed in a liquid to change the material properties, remove portions of the conductive layer under the opening in the photoresist layer to form a line of conductive material on the substrate.
According to another aspect of the present invention, there is provided a semiconductor device made by using the methods described herein.
According to another aspect of the present invention, there is provided a composition for a photoresist consisting essentially of a matrix material, a sensitizer material, a solvent material, and a cross-linkable material that reacts with a stabilizing treatment to form molecular cross-links.